In a conventional stamping operation, a hole is produced in strip material by a punch which shears a slug from the material and pushes the slug into the opening in the die. The slug should remain in the die opening when the punch returns to its retracted position so that strip material can be fed and the punching operation repeated. Under some circumstances, the slug will have a tendency to adhere to the face of the punch when the punch returns to its retracted position from its extended position and will be carried with the punch. As a result, the slug remains on the face of the punch when the subsequent stamping cycle is carried out. This phenomenon, commonly referred to as "slug pulling" can be extremely troublesome and can cause breakage of the punch or other damage to the punch and die assemblies. In any event, the machine must be stopped if slug pulling occurs and the slug removed from the face of the punch before the machine can be restarted.
There are several systems which can be incorporated into the die assembly to prevent slug pulling. A vacuum can be applied behind the slug in the die opening so that the slug will be pulled from the face of the punch by the vacuum. Alternatively, compressed air jets can be provided in the die assembly which inject compressed air into the opening and cause the slug to separate from the punch. These systems are successful but are not, in general, desirable for the reason that they are wasteful of energy and have a polluting effect on the atmosphere of the room in which they are used. These systems require a continuously operating compressor or vacuum pump and are not energy efficient for that reason. Vacuum and compressed air systems have a polluting effect as a result of the fact that the air streams carry droplets of lubricant which is used in the operation of the compressors. U.S. Pat. No. 4,489,871 and the other U.S. patents cited above show a type of stamping machine having relatively small die and punch assemblies which move towards and away from each other during each operating cycle. It is not desirable to provide a vacuum system or compressed air system in machines of this type for the additional reasons that the die assemblies are relatively small and the die assembly moves during operation of the machine.
It is also possible to minimize slug pulling by designing the punch and die such that the punch clearance is extremely small, for example, 5 percent of the stock thickness or less. A punch and die assembly having a very small punch clearance results, however, in shortened punch life and requires more maintenance than is required if a generous clearance is provided, for example, 8 percent of the stock thickness. It is also possible to minimize slug pulling by beveling or otherwise providing contours on, the face of the punch but this system requires extra machining and results in higher manufacturing costs for the punches. Furthermore, it is not always successful.
The present invention is directed to the achievement of an improved die assembly having a static means therein which minimizes or entirely prevents slug pulling and which ensures that the slug produced during a stamping operation will be retained in the die opening when the punch returns to its retracted position.